There are known two general methods for accomplishing bending work on metal pipes, rods, or the like. In cold bending, the desired bending work is performed by giving to the work piece a bending moment that exceeds the elastic limit of the work piece at normal temperature. In hot bending, the bending work is accomplished by giving a bending moment to the work piece by heating it to a temperature that induces plastic deformation of the work piece. Cold bending is suited for relatively small diameter metal pipes or the like of which no high product precision is required, but it is not suited for relatively large diameter metal pipes or the like of which high product precision is required. Whereas, hot bending is suited for relatively large diameter metal pipes or the like of which high product precision is required, but fairly largescale equipment and a heating apparatus is required. Also, as work efficiency is poor, the working cost is high.
Attempts have been made to overcome these problems. One attempt has been recently developed and put to practical use as an improved method and apparatus for effecting bending work by hot bending. According to this improved method, a metal pipe or the like, to be bent, is first passed through a heating device, such as for example, a high frequency inductor. The high frequency inductor is capable of effecting high temperature heating over a limited area, with the end or a suitable middle part, of said pipe being clamped to an arm which is freely swingable and whose axis of revolution is located within the plane of said heating device. The arm also has a length that matches the bending radius of said pipe. While the pipe is being continuously driven straight forward, it is subjected to local heating to achieve a plastic deformation inducing temperature by said heating device, the heated portion of the pipe being so as to effect continuous plastic deformation in said heated area of said pipe while giving a bending moment to said pipe, thereby to accomplish the desired bending.
Referring first to FIG. 1, there is shown such a conventional bending apparatus. In the figure, reference numeral 1 designates a steel pipe to be bent, 2 a support block adapted to support the pipe end and formed integral with the means for continuously and straightforwardly propelling the steel pipe 1 and a pair of guide rolls 3 and 4, a heating device 5, such as an annular high frequency inductor which is capable of heating a limited area of the steel pipe 1 sidewise to a high temperature, a cooling device 6, integral with said heating device, a rocker arm 7, the pivotal shaft 8, of the arm 7 arranged such that its center resides within the plane of the heating device 5, and a clamp 9 fixed to the arm 7.
In operation of the apparatus just described, steel pipe 1 is first passed between guide rolls 3 and 4 and then further passed through heating device 5 as shown in the drawing. The pipe end is supported by support block 2 of the propelling means, and in certain applications, on end or a suitable middle portion of the steel pipe 1 is fastened to the arm 7 by the clamp 9. The steel pipe 1 is continuously fed straightforwardly by the propelling means while subjected to local heating by heating device 5 to a plastic deformation inducing temperature, and this treatment is immediately followed by cooling so as to effect continuous plastic deformation of the steel pipe 1 in its heated area while giving a bending moment to the pipe by the thrust of the propelling means under the guidance of the arm 7, thereby to accomplish the desired bending. Thus, according to this apparatus, the desired bending of steel pipe can be performed at high efficiency without requiring any elaborate thermal works. The same effect can be obtained by using this apparatus for bending of other types of metal pipes or metal strips. It is to be particularly noted that no bending mold is required and it is possible to bend the pipe at any desired radius of curvature.
According to this method and apparatus, the desired bending can be accomplished on solid metal materials very efficiently with high precision, but when hollow metal pipes are subjected to bending, there takes place in some cases a phenomenon detrimental to the product, such as flattening, flexing or buckling in the pipe, and this may cause a change of curvature increasing or reducing the bending radius of the pipe.
In recent years, pipelines are popularly used for the transportation of fluids, and the number of steel pipes used for such pipelines is increasing at a high rate. Also, more and more high precision is required for bending such pipes.
Thus, demand has been voiced in the industries for development of a method and apparatus which are capable of effecting bending metal pipes such as steel pipes with higher precision than is attainable with the presently available techniques.
In view of the above, the present invention has for its object to provide a method and apparatus which are capable of bending long metal materials such as pipe, bar and rod with high efficiency and high precision.
It is another object of the present invention to provide a method and apparatus which are capable of bending steel pipes used for constructing pipelines for fluid transport without causing any undesirable phenomenon such as flattening, flexing or buckling.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the appended drawings.